The book does not include a family tree as such; that is something you will have to put together yourself. I am assuming that your assignment is to make a family tree for the Creighton family, and to do this you would have to start with a list including the names of each member of the family, and organize them by generation.
At the top of your family tree, you would have Matthew and Ellen Creighton. Matthew and Ellen each have at least one sibling, who can be put on the same line with them, since they belong to the same generation. Matthew's sibling gave birth to Eb Carron, who would be on the second line of the tree with the Creighton children, the next generation. In like manner, Ellen's sister had a son, Wilse Graham. Wilse would belong on the same line as Eb Carron and the Creighton children as well.
Matthew and Ellen Creighton have a total of nine children, four of whom are dead and buried on Walnut Hill at the beginning of the story. All of these children, living and dead, would appear on the same line as Eb and Wilse on the family tree. Although their birth order is not specifically stated, it appears that either John or Mary is the oldest Creighton child. Mary, a beautiful young lady, was killed two years back in a tragic accident. John, the eldest son, is married to Nancy, whose name would be placed next to his on the family tree. Below them would be their two children, a boy and a girl, the only ones belonging to the next generation in the family.
Bill Creighton would probably come next after John and Mary, followed by Tom, who, like his cousin Eb, is eighteen when the story begins. Following Tom would be Jenny, who is born four years after Tom, and when she marries Shadrach Yale, his name would be added next to hers. Following Jenny, I believe, are three little boys, all of whom died of polio the year Jethro was born. One of the boys' names is Nate, the others, I don't think, are given. Finally, there is Jethro himself, the youngest of the Creighton children, who is nine when the story begins.